The thermally sensitive recording method is the method to obtain recorded image by providing a thermally sensitive color developing layer mainly composed of colorless or pale colored electron donning type dye precursor and electron accepting type color developer on a substrate and by reacting said dye precursor and color developer instantly with heat using thermal head, thermal pen or laser beam, and is disclosed in Japanese Patent Laid-Open publication 43-4160 or Japanese Patent Laid-Open publication 45-14039. This kind of thermal recording method does not need a developing and fixing processes and very shape image can be obtained by simple equipment. Further, the equipment has the advantageous of relatively cheap, compact and maintenance free and noiseless. Therefore, this method is widely applied in the field of a facsimile, a printer, a recorder of measuring instrument, a labeling machine and for a bending machine for train tickets. Along with the expanding of the usage, the required qualities to a thermally sensitive recording medium are becoming more multiplex, for instance, a higher sensitivity, a stabilization of image and a multi coloration of image can be mentioned. Especially, the multi coloration of recorded image has a merit that the letters or patterns to be emphasized can be recorded by a different color tone from the other part.
As a multi chromatic thermally sensitive recording medium, following two types are known. That is, one is a laminated layers type which prepares plural color developing layers that develop different colors on a substrate and to form a recording image using variations of heating temperature or thermal energy, and another one is a single layer type which contains two or more colors precursors which develop different color in same color developing layer.
The multi layered type is characterized to laminate a high temperature color developing layer and a lower temperature color developing layer which develops color by lower temperature range or by lower thermal energy than the high temperature color developing layer, and multi colored by color erasing or color adding. Color developing mechanism in color erasing type can be illustrated as follows. That is, only lower temperature color developing layer develops color by lower temperature heating and at the higher temperature heating, a color erasing agent which has color erasing function to color developing line of the lower temperature color developing layer acts so that only high temperature color developing layer develops color. This method has an advantage that the color developing tone can be voluntarily selected, however, for the purpose to obtain enough color erasing effect, it is necessary to add large amount of color erasing agent, and too much adding of the color erasing agent deteriorate the preservability of the recorded image. Further, since a lot of thermal energy is wasted to fuse the color erasing agent, the problem that the recording sensitivity is deteriorated causes.
In the case of color adding type, a lower temperature color developing layer develops color by lower temperature and a high temperature color developing layer develops color by higher temperature, namely, since these color developing layers develop color simultaneously and colors of these two layers are obtained by mixing, it is necessary to restrict substantially the color of high temperature color developing layer to black. Further, since the black color image of high temperature color developing layer is obtained by mixing with color developing line of lower temperature color developing layer, the color of lower temperature color developing layer is fogged to black color image and the difference between developed color tone become unclear, in particular, the color mixing becomes remarkable at the outermost periphery and becomes difficult to obtain clear black color. Further, when exposed in the high temperature environment such as on a dashboard of car, following problems, namely, the high temperature color developing layer develops color and fogs on the developed color tone by lower temperature, or color becomes black, are caused.
In the case of single layered multi colored thermally sensitive recording medium, the use of micro capsule is known. In Japanese Patent Laid-Open Publication 60-242093, the art to contain different color developing components in two or more kind of micro capsules whose glass transition point are different. In Japanese Patent Laid-Open Publication 4-101855, the method to form a micro capsule is disclosed. That is, the solution prepared by dissolving dye precursor and wall forming material of microcapsule in water insoluble organic solvent, is emulsified and dispersed in hydrophilic colloidal solution, then the system is heated and the inner pressure of a reacting vessel is vacuumed so as to evaporate off the organic solvent and to form the wall of micro capsule.
However, in the case of above mentioned Japanese Patent Laid-Open publication 60-242093, when the dyes which develop different colors are micro capsuled separately, since these dyes are completely isolated by wall of capsule, the color developing sensitivity are deteriorated and the color isolation feature becomes bad. Further, the micro capsule obtained by the method disclosed in Japanese Patent Laid-Open Publication 4-101855, is easily crushed by pressure. The micro capsule of Japanese Patent Laid-Open Publication 9-76634 is illustrated as follows. That is, the micro capsule is prepared by making a dye as a solute, and oily solution whose solvent is organic solvent is emulsified and dispersed in aqueous solution and to form a wall film composed of polymer surrounding an oily drop. However, in the case of micro capsule which uses organic solvent, the color developing reaction of leuco dye with color developing agent is caused through organic solvent, and causes the problem of deterioration of the ground color or becoming dirty by friction. Still further, the micro capsule of Japanese Patent Laid-Open Publication 4-101855 containing solid state dye precursor is easily crushed and is inferior in ground color and resistance to friction, and is not sufficient to obtain a multi colored thermally sensitive recording medium having enough quality.
For the purpose to supplement the defect of the conventional micro capsule, in Japanese Patent Laid-Open Publication 9-102025 and Japanese Patent Laid-Open Publication 9-290565, the method to use complex fine particle obtained by complexing dye precursor with polymer compound composed of poly urea or polyurethane. However, although this method can dissolve the problem of capsule destruction, this method is not sufficient for the purpose to obtain clear lower temperature developed image and clear high temperature developed image.
Concerning above mentioned circumference, the subject of the present invention is to provide a multi colored thermally sensitive recording medium which can obtain clear color tone both is lower temperature color development and higher temperature color development, no fogging at lower temperature color developing tone and high temperature color developing tone and is excellent in color isolation.